Although protein-redox reactions provide the primary energy for ATP synthesis and all related metabolism, little is known about the mechanisms which provide for efficient long distance electron transfer in protein-protein complexes. Therefore, metal substituted (filled shell) photoactive heme proteins (e.g.: Zn(II)cyt c, Zn(II)cyt b5, Zn(II)Hb) will be used to investigate the rates and mechanisms of long distance electron transfer in strongly (noncovalently) bound protein-protein complexes. Specific systems of interest include Zn(II)cyt c Fe(III)cyt b5, Zn(III)cyt b5/Fe(III)cyt c, Zn(II)Hb/Fe(III)cyt b5, Zn(II)cyt b5/Fe(III)Hb, Zn(II)cyt b5,Fe(III)cyt c, Zn(II)Hb/Fe(III)cyt b5, Zn(II)cyt b5/Fe(III)Hb, Zn(II)cyt c/Fe(III)cyt c peroxidase, Zn(II)cyt c/Fe(III)cyt c1, Zn(II)cyt C/Fe(III) cyt b2, Zn(II)cyt c/Fe(III)cyt oxidase, and the corresponding homologs in which Zn(II) is replaced by a free base or Mg(II) porphyrin. A series of specific Hb derivatives in which the sequence has been altered by evolution will be studied to elucidate the role of specific amino acids in facilitating electron transfer. Detailed studies of the temperature dependence of electron transfer will be carried out for selected systems in order to clarify the role of nuclear tunneling in biological electron transfer.